1. Field of the Invention
The invention relates to a control apparatus and control method for a fuel cell.
2. Description of the Related Art
A polymer electrolyte fuel cell has an electrolyte membrane, two catalyst layers formed by sandwiching the electrolyte membrane therebetween, and a pair of diffusion layers formed on outer sides of the two catalyst layers. One diffusion layer in the fuel cell is supplied with a fuel gas including hydrogen, and the other diffusion layer is supplied with an oxidizing gas including oxygen. The diffusion layer to which the fuel gas is supplied is called a hydrogen electrode, or an anode; the diffusion layer to which the oxidized gas is supplied is called an air electrode, or a cathode.
Hydrogen supplied to the hydrogen electrode diffuses to the catalyst layer, and separates protons and electrons in the catalyst layer. Separated protons then pass through the electrolyte membrane along with water molecules and move to the catalyst layer on the positive electrode side.
On the contrary, oxygen supplied to the air electrode diffuses to the catalyst layer on the air electrode side, and water is generated through a reaction among protons, electrons, and oxygen. By connecting the air electrode and the hydrogen electrode to an external circuit (i.e., a conductor), electrons move from the hydrogen electrode to the positive air electrode, and are expended in reactions with the above protons.
To increase the amount of electricity generated in the fuel cell, an amount of oxidizing gas and fuel gas corresponding to the amount of electricity to be generated must be supplied to the air electrode and the hydrogen electrode, respectively. In general, air is used for oxidizing gas.
However, air includes oxygen and nitrogen. Since nitrogen is not used in the reaction on the air electrode side, in addition to accumulating in space on the air electrode side, the nitrogen also diffuses to the diffusion layer and electrolyte membrane, and ultimately passes to the hydrogen electrode side. Water generated on the air electrode side due to the reaction among protons, electrons, and oxygen also diffuses to the diffusion layer and electrolyte membrane, and ultimately passes to the hydrogen electrode side. Therefore, operating the fuel cell for extended periods of time increases the concentration of impurities, such as water vapor and nitrogen, unlike hydrogen in space on the hydrogen electrode side of the fuel cell. Patent documents related to a control apparatus for a fuel cell include Japanese Patent Laid-Open Publication No. 2002-353837, Japanese Patent Laid-Open Publication No. 7-169488, Japanese Patent Laid-Open Publication No. 2003-331889, and Japanese Patent Laid-Open Publication No. 9-259913.
An increase in the concentration of impurities other than hydrogen on the hydrogen electrode side impedes an increase in hydrogen concentration, which in turn impedes an increase in the amount of electricity generated. For this reason, a conventional polymer electrolyte fuel cell has been provided with an exhaust valve on a downstream side of a fuel gas passage of the hydrogen electrode, in order to discharge post-reaction fuel gas (hereinafter referred to as “fuel off-gas”) on the hydrogen electrode side.
Such an exhaust valve (as described, for example, in Japanese Patent Laid-Open Publication No. 2002-353837) opens when the fuel cell is activated, and is used to discharge impurity gas on the hydrogen electrode side, leading to an increase in the hydrogen concentration on the hydrogen electrode side.
After activation of the fuel cell, the exhaust valve is opened and closed according to a predetermined sequence to discharge impurities on the hydrogen electrode side and maintain a generated electricity amount.
However, the exhaust valve of a fuel cell system (where impurities are discharged to increase the hydrogen concentration by opening and closing the exhaust valve during activation) may freeze under a low temperature. Since time is required to defrost the frozen exhaust valve, the fuel cell cannot be activated in a short period of time. Furthermore, fuel efficiency is not always satisfactory in a fuel cell system where the exhaust valve is opened and closed during operation to maintain the generated electricity amount, because hydrogen may be discharged along with impurities.